linux-kernel - Re: [RFC PATCH v1 1/1] arm64: add compile-time test into is_compat

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Date: Thu,  4 Jan 2024 22:16:39 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Leonardo Bras <leobras@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Teo Couprie Diaz <teo.coupriediaz@....com>,
	Mark Rutland <mark.rutland@....com>,
	Mark Brown <broonie@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Guo Hui <guohui@...ontech.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] arm64: add compile-time test into is_compat_task()

On Thu, Jan 04, 2024 at 09:43:56PM +0100, Arnd Bergmann wrote:
> On Thu, Jan 4, 2024, at 20:24, Leonardo Bras wrote:
> > Currently some parts of the codebase will test for CONFIG_COMPAT before
> > testing is_compat_task(), probably in order to avoid a run-time test into
> > the task structure, while other parts of codebase will just test even when
> > the option is not compiled in.
> >
> > Since is_compat_task() is an inlined function, it would be helpful to add a
> > !CONFIG_COMPAT version of the helper, allowing compile-time optimization.
> >
> > With this, the compiler is able to understand in build-time that
> > is_compat_task() will always return 0, and optimize-out some of the extra
> > code introduced by the option.
> >
> > This allows optimizing-out code when the option is not selected, and
> > otherwise removing a lot #ifdefs that were introduced, making the code
> > more clean.
> >
> > Signed-off-by: Leonardo Bras <leobras@...hat.com>
> 
> This looks like a useful cleanup to me, 


Hello Arnd, thanks for reviewing! 


> with one change:
> 
> > ---
> >  arch/arm64/include/asm/compat.h | 5 +++++
> >  arch/arm64/kernel/ptrace.c      | 6 ++----
> >  arch/arm64/kernel/syscall.c     | 5 +----
> >  3 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/compat.h b/arch/arm64/include/asm/compat.h
> > index ae904a1ad5293..3cc61cbbb9062 100644
> > --- a/arch/arm64/include/asm/compat.h
> > +++ b/arch/arm64/include/asm/compat.h
> > @@ -100,6 +100,11 @@ long compat_arm_syscall(struct pt_regs *regs, int scno);
> > 
> >  #else /* !CONFIG_COMPAT */
> > 
> > +static inline int is_compat_task(void)
> > +{
> > +	return 0;
> > +}
> > +
> 
> I think this bit is not even needed as long as users
> include linux/compat.h rather than asm/compat.h, as there
> is already a macro definition in the common file:
> 
>  #define is_compat_task() (0)
> 

Oh, I was unaware of this macro. Thanks for pointing it out!

I just checked every use of is_compat_task() in the codebase for (arch == 
arm64 && non-arch code), and it seems like the file will either include 
linux/compat.h or another header which includes linux/compat.h. 

So it's safe to assume the macro will be available for every user.

I will send a v1 soon.

Thanks!
Leo